Material Dispersion

Material dispersion is a phenomenon in which different optical wavelengths propagate at different velocities, depending on the refractive index of the material used in the fibre core.
The refractive index of the glass varies according to the wavelength of the optical signal, different wavelengths travel at different speeds in the medium.
The effect of material dispersion is that the various wavelengths comprising the pulse travel at different velocities through the medium.
So, the pulse can spread over a distance simply due to the interaction of various wavelengths with the matter in the crystalline core, which causes some portions of a pulse to arrive earlier than other portions.
As the wavelength increases (and frequency decreases), material dispersion decreases.